The present invention broadly relates to a conduit assembly for use immersed in a liquid metal. It is particularly directed to a conduit assembly for interconnecting a pump discharge duct and a reactor coolant inlet duct in a pool-type, liquid-metal cooled, nuclear reactor.
The design of conduit assemblies for use in pool-type reactors is very demanding since there is a need for minimizing any stress resulting from thermal expansion and relative movement of the two components being interconnected. In addition, it must be capable of withstanding the calculated stress which would result from a seismic event having a specified magnitude. The problem in the design of such conduit assemblies is further complicated by the fact that the conduit is generally inaccessible for inservice inspection since it is submerged in a pool of liquid metal.
Various types of interconnection have been proposed for use or used in such an environment. For example, it has been suggested that the interconnecting conduit be provided with a bellows to allow for relative motion between the two components as well as thermal expansion of the conduit cell. The disadvantage of the bellows, however, is that it is not readily amenable to removal for inspection and repair if such should be required. Further, the flexing of the bellows could result in work hardening of the metal and subsequent failure of the bellows.
Perhaps the simplest design utilizes a plurality of permanently installed conduits for each pump. The conduits are formed with one or more expansion loops to provide for thermal expansion and the differential movement of the components so interconnected. A disadvantage of this design is, of course, that the pipes cannot be inspected, should such be required, without draining the sodium from the tank. Further, the constant flexing of the expansion loops could result in cracking of the conduit.
Another design concept utilizes a spherical joint at each end of an elongated conduit. The joint comprises a spherical ball in one portion of the conduit in mating engagement with a spherical seat in another conduit to provide for pivotal motion of the conduits. In addition, a cylindrical portion may be provided having a diameter similar to that of the spherical ball to provide for some axial movement. This type of an arrangement may be constructed to be amenable to removal for inspection or repair. A disadvantage of this concept is that it permits a significant amount of leakage; typically from 1 to 10% of the fluid being pumped through the conduit leaks out through the joints. This leakage results from the necessity of providing clearance between the mating metal parts such that they may be remotely installed in or removed from the pool of sodium. The leakage can be minimized, however, only at the expense of maintaining very close tolerances, which is of course both expensive and difficult for large conduits which may be as much as several feet in diameter. In addition, the more closely the tolerances are held to minimize any gaps and leakage, the greater is the possibility that binding between the parts will take place. Any such binding would, of course, defeat the purpose of having movable joints.